In 2016, the independent democracy non-profit Freedom House scored the United States electoral process as 11 out of 12.
A 2014 university study found only 31 cases of voter fraud out of 1,000,000,000 ballots cast since 2000.
" any measure, voter fraud is extraordinarily rare" - Report by NYU Brennan Center For Justice.
Elections are run independently at the local level, by local citizens, under the supervision of both parties. Read more here

If someone claims otherwise, demand proof.